For months now, the most useful credit-analysis tool for European investors has been a very simple one: a map. Corporate borrowing costs are being determined almost entirely by where a company happens to be based, even if its businesses are pan-European or global. And some recent bond deals show that the difference between north and south is becoming extreme. Investors weighing up what the European Central Bank will do next should pay attention.

Northern European borrowers are increasingly able to lock in extremely cheap financing - including at negative real rates - while southern companies are left out in the cold. The numbers speak for themselves: German engineering giant Siemens is paying investors a coupon of just 0.375% for a two-year bond, a record low, while Spanish bank Santander had to offer a coupon of 4.375% for the same maturity.

Northern borrowers are increasingly tapping longer maturities too given the low cost: German car maker Volkswagen issued a 10-year 2.375% bond last week, the first bond from an auto maker with a maturity of more than seven years since 2007, according to a banker who worked on the deal. Friday, Nordic telecoms operator TeliaSonera launched a 15-year euro bond - a relative rarity - paying a coupon of just 3%.

On bank loans, a similar picture emerges. In June, rates for German bank loans to non-financial companies of more than €1m and with a maturity of one to five years came in at 3.37%, according to ECB data. A similar loan in Spain cost 5.7%. This is perhaps of even greater importance given corporate Europe's reliance on banks for funding, in contrast to the US where capital markets play a greater role.

The longer these divergences persist, shaping corporate capital structures, the more they will give Northern European companies a structural advantage over their Southern peers, further hampering efforts to narrow the imbalances between eurozone nations. Far more than the issue of sovereign borrowing costs, it is likely to be this continuing distortion that is a big worry for the ECB as it means the monetary transmission mechanism is broken.

But the ECB is talking only about buying short-dated government debt - potentially exacerbating these trends whereby long-term financing is expensive or unavailable in Southern Europe. And another factor will complicate the ECB's efforts to repair the transmission mechanism. It still isn't clear whether further aid for Spain or Italy might trigger downgrades from the ratings firms. Spain is on the brink of "junk" status at Moody's - and many investors are shunning Spanish corporate bonds for that reason.

If eurozone intervention does lead to mass downgrades for Southern European companies, then the north-south divide will only become further entrenched.